The present invention relates to an auxiliary power assisted master cylinder arrangement for a vehicle brake system. Various constructions of master cylinder arrangements operative to generate a controlled brake pressure in associated wheel brake cylinders are already known, even such equipped with a valve arrangement connected to a source of auxiliary power and adapted to operate the master cylinder piston, wherein the valve arrangement is in turn operable through the brake pedal under the control of a device including mechanical links which effect a control of the master cylinder(s) in the event of a failure of the auxiliary power.
A power-assisted master cylinder arrangement of this type is known in the form of a brake slip control apparatus as disclosed in German published and examined patent application DE-AS No. 2,366,108. It includes a master cylinder arrangement embodying three master cylinder units disposed in parallel. A booster arrangement is inserted in front of such master cylinder units. For each master cylinder unit, the booster arrangement possesses a booster piston to mechanically operate the associated master cylinder piston in any braking situation. The ends of the booster pistons which are not engaged with the master cylinder pistons extend in a sealed booster chamber into which pressure fluid may be admitted through a booster valve. The booster valve is operable by the brake pedal through a valve member including an actuating rod. In normal operation of a brake, actuation of the brake pedal will open the booster valve and pressure fluid will be admitted to the booster chamber. The supplied controlled pressure will act upon the secondary ends of the three booster pistons which will displace the associated subsequent master cylinder pistons of the master cylinder units and thereby effect a static activation of the wheel brake cylinders. In the event of a failure of the auxiliary power, a cross link of the actuating rod will directly engage the secondary ends of the booster pistons to thereby effect displacement of these pistons and consequently an (auxiliary) static activation of the wheel brakes. In the anti-skid control operation of a wheel brake cylinder and during normal operation of the brake, i.e., with auxiliary power being available, the end of the respective booster piston on the side close to the master cylinder piston will be acted upon by pressurized fluid against the pressure acting on the secondary end of the piston in the booster chamber, such that the booster piston will be returned in spite of a pressure existing in the booster chamber, whereby the associated master cylinder piston will be released to assume its brake release position and the brake pressure supplied to the controlled wheel brake cylinder will be decreased. In view of the tandem design of the booster and master cylinder arrangement, the known brake slip control apparatus includes relatively many single parts and sealing points. The structure is relatively incompact and complicated. The susceptibility to failure in operation is accordingly high. In particular, the known arrangement requires that a large volume of auxiliary power be made available to compensate for the increased volume of the booster chamber resulting from a failure of a brake circuit and to ensure application by pressure of the remaining intact booster pistons in a satisfactory manner. Also, to build up a counterpressure at the booster in the anti-skid control operation, a comparatively large amount of auxiliary power is necessary, so that the auxiliary power supply system has to be amply dimensioned in its entirety. In particular, a relatively large fluid accumulator and/or a control pump with a relatively high delivery have to be provided. Because in practical operation of a vehicle the anti-skid control operation occurs only rarely, the above-mentioned disadvantages have a pronounced impact.